Advances in semiconductor technology have allowed for increasingly sophisticated systems to be built in smaller packages. Small devices may contain circuitry to connect to the Internet and perform some useful function such as sensing temperature, heart rate, or acceleration, or controlling a camera, refrigerator, door lock, or automobile sub-system. A huge number of such connected devices will exist in the Internet of Things (IoT).
Connections without wires (Wireless connections) are often preferred to connections with wires to minimize installation costs. Most of these connected devices will be battery powered, but some will derive energy from external electro-magnetic radiation (EM) such as radio waves. Energy harvesting circuits extract energy from an external EM source to power a circuit or recharge a battery.
Near-Field Communication (NFC) circuits have receivers placed very close to transmitters, such as within a few centimeters or almost touching. However, most connected devices are not placed so close to receivers. NFC has a higher energy transfer than does far-field. Thus NFC power harvesting is not available for many connected devices since they are positioned too far from transmitters for near-field effects.
FIG. 1 shows a far-field energy harvesting application. Hub or base station 142 transmits Radio-Frequency (RF) waves to connected devices 140. Internet Protocol (IP) packets may be encoded and transmitted by the RF waves. Connected device 140 may transmit return packets back to base station 142 that include acknowledgements and sensor data.
Connected device 140 may have a small battery or capacitor that is recharged from energy received from base station 142. RF wave energy is converted to Direct Current (DC) power by an energy harvesting or RF-to-DC converter circuit in each of connected devices 140. When RF energy from base station 142 is received, connected devices 140 may wake up and perform various programmed functions.
The distance from base station 142 to connected device 140 varies but is typically well beyond the near-field boundary, and far-field energy transfer is much less efficient and lossy than for near-field. The theoretical energy transferred is dependent on the RF frequency, transmitted power, and distance between base station 142 and connected device 140. For example, a 900 MHz RF transmission from base station 142 results in only 28 μW (micro-Watts) of a 74 mV signal received by a 50-Ohm antenna on connected device 140 that is placed 10 meters from base station 142.
Dickson charge pumps and other rectifiers have been used as energy harvester circuits. However, the input sensitivity and power conversion efficiency are insufficient for many applications. Transistor threshold voltages may consume too much of the small available input signal from a tiny antenna. Diodes or diode-connected transistors have too large of a voltage drop across them, thus consuming too much of the small input signal.
FIG. 2 is a block diagram of a connected device that obtains power from an external RF transmission. Connected device 140 has base band processor 102 that executes programs or routines in EEPROM 104, such as to read sensor 116 through interface analog-to-digital converter (ADC) 106. Base band processor 102 embeds sensor data into an IP packet that is sent by transmitter 112 through antenna 122 to an external base station. Packets from the base station received by antenna 122 are received by receiver 108 and processed by base band processor 102.
RF-to-DC converter 110 receives the signal from antenna 122 and generates a DC voltage to charge capacitor 114. Capacitor 114 acts as a battery to power all components of device 120 and sensor 116. Since the amount of power received by antenna 122 is very small for RF waves, RF-to-DC converter 110 must be highly efficient and very sensitive. Low ripple on the output is desirable so that a smaller capacitor 114 may be used.
What is desired is a RF-to-DC converter for low-power applications such as for connected devices. A RF-to-DC converter that is highly efficient yet very sensitive is desirable. An energy-harvesting circuit using a standard complementary metal-oxide-semiconductor (CMOS) process that can convert small voltages generated by RF waves that are not near-field is desirable.